We have detected in crude extracts a nuclear activity which promotes the binding of Rev to the Rev-response element (RRE). This activity should be crucial for the in vivo function of Rev. This activity either cooperatively binds to the RRE with Rev or catalyzes the binding of Rev to the RRE. Almost certainly the activity is protein. Chromatographic analysis of the extracts has resolved at least three and possibly as many as five separate activities. Currently we are purifying the separate activities and cloning them. In parallel with these studies, we have detected the existence of cell-type specific cellular factors by studying the replication kinetics in different cell types of human immunodeficiency virus (HIV) proviruses with RRE mutations. In different cell types, HIV proviruses with different RRE mutations replicate with drastically different kinetics. These data are the first reported indication that the RRE may contribute to cell-type specific viral tropism. In related work we have studied the interaction of the Rev protein with the RRE. We have found that RRE elements determine the binding of Rev molecules subsequent to the first binding event. This precludes models for Rev/RRE binding which hold that all of the specificity is in the first binding event and that subsequent multimerization of Rev along an RNA molecule involves only non-specific recognition of RNA by Rev protein. This work has also identified RRE molecules that tightly bind Rev but that do not permit more than two Rev molecules to bind. Functional analysis of theses mutations will assess the requirement of multimerization for function.